In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form images or characters. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using a plain paper as the recording medium, non-contact with printed images or characters, etc.
In recent years, as an ink for ink-jet printing, there have mainly been used pigment-based inks prepared by dispersing a pigment, a polymer, etc., in water, from the viewpoints of good water resistance and weather resistance thereof.
As a method of ejecting an ink in the ink-jet printing methods, there are known a piezoelectric method utilizing a mechanical energy and a thermal method utilizing a thermal energy.
The ink containing a polymer tends to suffer from ejection defects of ink droplets owing to increase in viscosity of the ink in ejection nozzles when an interval between the ink droplets ejected is increased during inkjet recording. To avoid occurrence of the defects, it is preferable to adopt the thermal ink-jet printing method having a higher ink ejection force of the ink droplets than that of the piezoelectric ink-jet printing method.
On the other hand, in the thermal ink-jet printing method, since a thermal energy is applied to an ink, there tends to occur such a phenomenon that contaminants such as soot are deposited onto a heater portion of a thermal print head, i.e., so-called “kogation” (deposition of residues on a surface of a resistive element).
For example, in JP 2010-138297A, it is described that in a recent thermal ink-jet printing method, there is such a tendency that a temperature of a surface of a resistive element used therein tends to be elevated for improving a quality of ink images produced, and therefore the kogation in the thermal ink-jet printing method tends to pose significant problems because it tends to induce deterioration in thermal efficiency and ejection efficiency of an ink, as well as deterioration in quality of ink images owing to ejection defects of ink droplets and decrease in ejection speed thereof.
JP 2010-138297A discloses a process for producing a water dispersion for thermal ink-jet printing which is excellent in ejection property, in which an anionic group of a polymer containing a constitutional unit derived from a methacrylic acid ester is excessively neutralized using a volatile base upon a dispersing treatment of particles of the polymer, etc., to enhance an ejection property of an ink in a thermal print head, and the volatile base is removed after the dispersing treatment, to maintain a good durability of the thermal print head.
In addition, JP 2004-107453A discloses an ink set suitable for thermal ink-jet printing which is excellent in color balance, etc., and capable of obtaining high-quality ink images. The ink set includes at least two kinds of ink compositions containing a pigment dispersed with an anionic substance, and a reaction solution capable of reacting with the ink compositions to coagulate the ink compositions, and the pigment contained in the ink is kept substantially identical in particle size before and after the reaction.